Multiple light-harvesting II polypeptides from maize mesophyll chloroplasts are distinct gene products

The major light-harvesting complex of photosystem II in higher plants is kn own as LHCII. It is composed of a number of chlorophyll-binding proteins sh aring epitopes with each other. The number of apoproteins resolved by fully denaturing sodium dodecylsulfate polyacrylamide gel electrophoresis varies in different species. In order to know if this heterogeneity is caused by the expression of a number of homologous genes or if it is the product of p ost-translational modifications, we have resolved the six major apoproteins of Zea mays LHCII. Each protein is purified to homogeneity, subjected to d irect protein sequencing and the sequences compared with those deduced from lhcb genes in maize and other organisms. All of the six proteins an distin ct gene products, since they show differences in their primary structure. T hree apoproteins are identified as products of type I lhcb genes and one ea ch as type II and type III gene products, A sixth protein does not fit the requirements for any of the lhcb genes so far cloned and is therefore proba bly the product of an lhcb gene type not yet described. Our results clearly show that the major source of LHCII protein heterogeneity is the expressio n of many lhcb genes. Fractionation of maize LHCII by non-denaturing flat-b ed isoelectric focusing resolves at least five major isoforms showing disti nct differences in their polypeptide composition and also differing in thei r spectroscopic properties, thus suggesting that individual Lhcb gene produ cts have distinct pigment-binding properties. (C) 1999 Elsevier Science S.A . All rights reserved.